An image display apparatus such as a PDP or a DMD, that performs a binary control of light emission and non-emission, typically uses a subfield method for intermediate gradation display. The subfield method uses a plurality of subfields weighted with the number of light emission or the amount of light emission to divide a single field by temporal decomposition, thereby performing a binary control of each pixel for each subfield. In other words, each subfield has its predetermined luminance weight, and the sum of the weights for emitting subfields determines the gradation level.
FIG. 12 shows an example configuration of a subfield in a conventional PDP. In this configuration, a single field is divided into eight subfields (SF1, SF2, . . . , and SF8), where respective subfields have luminance weights (1, 2, 4, 8, 16, 32, 64, and 128). Each subfield is composed of setup period T1 for preliminary discharge, address period T2 during which data for light emission or non-emission is written for each pixel, and sustain period T3 during which pixels with light-emitting data being written are made to emit light all at once. Combining these subfields in various ways for emitting light can produces 256 level gradation of “0” level through “255” level. For example, gradation level “7” is presented by emitting SF1, SF2, and SF3 having luminance weights 1, 2, and 4, respectively, and gradation level “21” is presented by emitting SF1, SF3, SF5 having luminance weights 1, 4, and 16, respectively.
In such a display apparatus that uses the subfield method for displaying multilevel gradation, it is known that false contours (dynamic false contours) appear and deteriorate the image quality while motion pictures are displayed. The dynamic false contours are described hereinafter. In the same way as the above, a single field is assumed to be divided into eight subfields (SF1 through SF8) respectively weighted with (1, 2, 4, 8, 16, 32, 64 and 128). AS shown in FIG. 13, a case is described where image pattern X moves on the screen of PDP 33 horizontally. Image pattern X has region P1 with gradation level “127” and region P2 with gradation level “128.” FIG. 14 is a view in which image pattern X is developed to subfields, where a horizontal axis corresponds to a horizontal direction on the screen of PDP 33, and a vertical axis corresponds to a time direction. In addition, hatched areas in FIG. 14 show non-emitting subfields.
When image pattern X remains stationary, as shown in FIG. 14, a viewer's viewpoint is also fixed to screen position A, and thus gradation levels “127” and “128,” which are pixel-original gradation, are perceived. However, when image pattern X moves to the left, the viewpoint also moves to the direction of screen position B-B′, and thus the non-emitting subfields in region P2 and region P1 are viewed. Consequently, gradation level “0”, i.e., a dark line, is perceived. Reversely, when image pattern X moves to the right, the viewpoint also moves in the direction of screen position C-C′, and thus light-emitting subfields in region P1 and region P2 are seen, where gradation level “255,” i.e., a bright line, is perceived. In either case, the gradation levels are largely different from the original gradation level “127” or “128”, and thus are perceived as contours. In this way, false contours, in spite of their small change in gradation levels, occur where the pattern of light-emitting subfields largely changes. For example, if subfields weighted as above-mentioned are used, also in cases where the luminance gradation levels of the adjacent pixels are “63” and “64,” “191” and “192,” or the like, false contours are prominently observed. Such a phenomenon is called a false contour noise, causing image quality to deteriorate.
Conventional methods for suppressing dynamic false contours include the following.
For example, convert the gradation level of an image signal to a “first gradation level” where dynamic false contours are unlikely to occur, and to its “intermediate gradation level,” and then diffuse an error caused by the conversion to the surrounding pixels to interpolate the skipping of gradation levels. Next, if the converted gradation level is “intermediate gradation level,” round up or round down to the nearest “first gradation level.” Repeat rounding-up and rounding-down alternately by dot, by line, and by field to present averagely “intermediate gradation levels.”
However, such a method has the following problems. Namely, the number of gradation levels inevitably decreases near a gradation level at which large dynamic false contours occur. In other words, suppressing dynamic false contours decreases the number of gradation levels, causing a visually rough image, while securing a desired number causes dynamic false contours to occur.